Ion implantation is a process by which atoms or groups of atoms are ionized to assume a net electrical charge by losing or gaining electrons so that the charged atoms can be excited to high speeds by an electrical potential and accelerated into a target sample. The target sample is bombarded with ions which are driven deeply into the surface thereof and distributed therein. This process is used to achieve an intimate mixing of incompatible materials to realize valuable new properties. Ion implantation is commonly used in the semiconductor industry for doping silicon to alter its electrical properties or in the treatment of metal, ceramic or polymeric surfaces to affect the hardness, optical properties, corrosion resistance, electrical conductivity, and other characteristics of the material.
Conventional ion implantation techniques utilize an ion source to accelerate a beam of ions toward a target sample for implantation therein. However, the use of an ion beam confines the implantation of ions to a line-of-sight region such that the target sample must be moved about in a steady, controlled manner during exposure to the ion beam in order to provide uniform implantation of all surfaces of the target sample. Such manipulation of the target sample, often provided via computer control techniques, can be very time consuming and expensive.
In order to overcome the problems associated with ion beam implantation methods, ion implantation has also been accomplished by generating a gaseous plasma of ions surrounding the target sample. For example, U.S. Pat. No. 4,764,394 discloses a method and apparatus for ion implantation utilizing a plasma ion source to create a steady state, gaseous plasma of ions to surround a target sample for implantation therein. Using the process of that patent, a negative voltage pulse is applied to the target sample to create an implant voltage which attracts and implants ions from the surrounding plasma into the target sample. Since plasma source ion implantation generates a plasma of ions surrounding the entire sample target for implantation therein, the process is not limited to line-of-sight implantation and therefore facilitates ion implantation of all exposed surfaces of a non-planar target sample without manipulation thereof. The referenced U.S. patent also discloses the use of multiple electrical pulses applied to the target sample in rapid succession for performing a series of ion implantations until a desired concentration of implanted ions is achieved within the target sample.
Although the plasma source ion implantation process disclosed in U.S. Pat. No. 4,764,394 provides significant improvements in ion implantation of three-dimensional materials, it is impracticable to use this process for the implantation of metal ions because high density metal ions tend to precipitate from the steady state ion plasma between implant voltage pulses. These ions become deposited onto the surface of the target sample and form a thin film of metal surface coating thereon. The resultant metal surface coating is not distributed into the surface of the target sample, thereby creating a barrier to any subsequent implantation. Thus, present ion implantation methods designed for implantation of metal ions are limited to ion beam applications, necessarily embracing the inherent deficiencies associated therewith.